1. Field of the Invention
The present invention relates to a neutralization apparatus having a minute electrode ion generation element, and more specifically, to a technique of neutralizing static electricity caused on an object surface, and a neutralization apparatus having a minute electrode ion generation element used for easily eliminating static electricity constituting a problem in various manufacturing processes.
2. Description of the Prior Art
The occurrence of static electricity in manufacturing processes can possibly result in reducing productivity and yields or causing electrical problems. Therefore, the neutralization technique of neutralizing and eliminating static electricity on an object surface by adhesion of positive and negative bipolar ions has been widely employed as an important technique for active control of static electricity. The neutralization technique with use of bipolar ions has been discussed in detail heretofore (see Non-patent Document 1) and commercialized by many manufacturers so far. Such neutralization apparatuses have widely been used in manufacturing processes of semiconductors, plastics, liquid crystals, etc.
A general configuration of neutralization apparatuses is such that an electrode for generating bipolar ions and a power supply, and a gas flow generating device for carrying the generated ions to an object are combined. For generation of positive and negative bipolar ions, air ionization by corona discharge or soft X-rays etc., is employed. The generated positive and negative bipolar ions are carried by gas flow or an electrostatic field etc., and adhere to an oppositely-charged physical object, thereupon reaching neutralization of its static electricity.
For apparatuses for eliminating static electricity, bipolar ion generating devices having a needle-type or wire-type electrode and employing corona discharge are most frequently used. This kind of ion generating device is described in detail in, for example, Non-patent Document 1, and an example of its configuration is shown in FIG. 12. In the device, gas molecules are ionized in the vicinity of a distal end of a discharge electrode 21, so that a large amount of ions are generated. In order to generate more or less the same number of positive and negative ions, application of a positive and a negative direct current voltage to different discharge electrodes respectively as disclosed in Patent Document 1 or application of an alternating current voltage as disclosed in Patent Document 2 are carried out. The positive and negative bipolar ions thus generated adhere to a charged physical object with Brownian movement in the course of being carried by gas flow, thereupon changing a surface potential of the object. As for adhesion probability of ions to a charged physical object in a circumstance where positive ions exist in an equivalent number to negative ions, adhesion probability of ions having a polarity opposite to an electric charge of particles exceeds adhesion probability of ions having the same polarity as particles. As a result, adhesion reaction between the positive and negative bipolar ions and the physical object brings the object surface into an uncharged state.
Herein, ion concentration is a parameter that determines a speed with which static electricity is neutralized, that is, the neutralization speed. Accordingly, in manufacturing processes requiring speedier neutralization, a device capable of properly balanced generation of positive and negative bipolar ions in higher concentrations is demanded.
For generation of positive and negative bipolar ions for the purpose of neutralization, a variety of electromagnetic waves can also be used. Generally in a method for generating positive and negative bipolar ions with use of electromagnetic waves, an electric charge of ionized gas molecules is conserved. Therefore, the method has a feature that for each polarity ion concentration ratio, ion balance is kept at more or less the same number between positive and negative ions. For example, nitrogen or other impurity molecules in the air are ionized by irradiating air with soft X-rays, whereupon positive ions and electrons are generated. Since a presence time of electrons is very short, oxygen, moisture, and other impurity molecules, etc., in the air are united with the electrons, whereby negative ions are formed. Consequently, generation of bipolar ions containing roughly the same amount of positive and negative ions becomes possible. Devices of this kind are described in Non-patent Document 1 and Patent Document 3, for example.
Other than the above, use of vacuum ultraviolet rays or radiation as electromagnetic waves is also possible, which is disclosed in Patent Documents 4 and 5, respectively.
In the method employing those electromagnetic waves, more powerful electromagnetic waves are necessary in order to meet a demand for generating the foregoing high concentration ions. However, there is a restriction that use of neutralization apparatuses employing radioactive substances that have the strongest energy is allowed only in a licensed facility and only by a person with a handling permit for radioactive substances. Further, even when the aforementioned conditions are satisfied, special care for safety control and storage to eliminate effects on human health involved in the use of radioactive substances must be taken. Similarly, it is necessary in neutralization apparatuses using vacuum ultraviolet rays and soft X-rays to take measures to ensure safety as irradiation energy is higher.
An air discharge voltage (ionization potential) differs between positive and negative ions in the aforementioned positive and negative bipolar ion generation by corona discharge. Thus, control of ion balance is generally difficult. In a form of applying a direct current voltage to a plurality of electrodes, for example, respective discharge voltages need to be controlled separately. In a form of using an alternating current voltage, a center voltage in a waveform needs to be offset etc. In order to control ion balance of positive and negative ions generated by corona discharge, there has been proposed a technique for conducting balance control by providing an ion balance control circuit separately as described in, for example, Patent Document 6 and a method etc., by regulating gas flow for positive and negative ions separately as described in Patent Document 7.
However, none of the methods described above can be a drastic solution to obtain speedy neutralization characteristics with stability for a long period of time. Therefore, development of a technique for generating high concentration ions in a well balanced manner has been demanded.
Another problem in the neutralization apparatus by corona discharge is abrasion of electrodes and buildup of dust etc., associated with long-term operation. They not only become a cause of trouble such as a short circuit between electrodes, static noise, etc., but also affect neutralization performance greatly due to changing the ion balance. In particular, a discharge voltage needs to be increased in a needle-type electrode in general use in order to produce higher concentration ions. In that case, reactive species of ozone and oxygen are generated in high concentrations, so that deterioration of the electrode is found more noticeably. To solve these problems, materials for the needle-type electrode which are low in deterioration (Patent Document 8) etc., have been proposed. However, the buildup of dust and deterioration are unavoidable in the discharge method such as a needle-type electrode in which a high voltage is required and thus an electric charge is concentrated locally. Accordingly, there has been demanded development of a bipolar ion generation element which can produce ions efficiently at a lower voltage, has a material or structure that resists buildup of dust etc., and deterioration and assumes a form in which replacement and maintenance thereof are simple and safe even if deteriorated.
On the other hand, an ion generation element with a configuration that a discharge electrode arranged in one direction on a plane and having minute protrusions is arranged on a dielectric body in order to improve maintainability is described in Patent Documents 9, 10, 11, and 12 as a use of a copier etc., for the purpose of charging and diselectrifying a drum in the vicinity of the ion generation element. Neutralization of a physical object disposed in a position away from the ion generation element by employing the technique described in those Patent Documents, which is different in usage from the latter, is difficult since the ion balance is disrupted due to differences in physical characteristics between positive and negative ions. Further, in the technique as described in Patent Documents 10, 11, and 12, control of the ion balance only by waveform control of a voltage is difficult. For the aforementioned reasons, such devices cannot be put into practical use as neutralization apparatuses in manufacturing processes.    Patent Document 1: Japanese Patent No. 2520840    Patent Document 2: Japanese Patent No. 2627585    Patent Document 3: Japanese Patent No. 2951477    Patent Document 4: Japanese Patent No. 2598363    Patent Document 5: Japanese Patent Pre-Publication No. H8-190993    Patent Document 6: Japanese Patent No. 3471511    Patent Document 7: Japanese Patent No. 2646020    Patent Document 8: Japanese Patent No. 3078819    Patent Document 9: Japanese Patent No. 2665903    Patent Document 10: Japanese Patent Pre-Publication No. 2003-323964    Patent Document 11: Japanese Patent Pre-Publication No. 2003-249327    Patent Document 12: Japanese Patent Pre-Publication No. 2002-237368    Non-patent Document 1: Ionizer and Charge Eliminating Technique, Supervising Editor, Yuji Murata, CMC Publishing Co., Ltd., 2004